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Improvement of Lactic Acid Production in Ultrafiltered Milk by the Addition of Nutrients

Department of Food Science, Cornell University, Ithaca, NY 14853

ABSTRACT

In 4 h at 32°C, mesophilic lactic acid bacteria produced .74% acidity in 3.3:1 retentate of ultrafiltration diluted to 2:1 with permeate. When diluted with distilled water instead of permeate, .56% acidity was produced. In 2.4:1 retentate containing .5% yeast extract, .95% acidity was produced compared with .84% in the control (retentate without yeast extract). The B vitamins, lactose, and minerals (NaCl, MgSO₄, FeSO₄, and MnCl₂) had little or no effect. With another lactic starter, 22 amino acids together at .04 mg/g each in 2:1 retentate improved acid production (.72% compared with .61% for the control). Nonessential amino acids as a group were more stimulatory than essential amino acids. Cysteine and cystine at .04 mg/g each were as stimulatory as all amino acids together and all amino acids minus cysteine and cystine. Reducing agent dithiothreitol at .05 mg/g was as stimulatory as cysteine and cystine. Sulfur-containing compounds (methionine and Na₂SO₄) had no effect, suggesting that the stimulatory effect of amino acids is via the reducing power of cysteine and cystine as well as nutritive value of other amino acids.

¹ Present address: Dairy Science Department, South Dakota State University, Brookings 57007.

² Present address: 5548 Hamlet Lane, Fort Myers, FL 33907.